And finally tonight, the monsters of mississippi making swamps look like a real life jurassic park. Giant alligators weighing more than 700 pounds. How did they get so big? Abc's gio benitez went into the swamp to find out. Reporter: They are monsters that can grow to epic proportions. Beth caught this 723 pound alligator. You're a parallel by day and alligator hunter by night. The alligator squeen. Reporter: Dustin, another gator animal. An animal, 700 pounds, something that can eat you, you pretty exciting. Reporter: They say hunting helps control an exploding gator population. Why are they so big? Because for years nobody was allowed to hunt them so the gators kept growing and growing. Alligators growing well into old age and some grow to 19 feet. That's the length of an average rv so we set out to find them. You don't want to spook them. Reporter: After hours of watching them hide under water, this gator showed his face. I hear his now. He's mad. Reporter: While beth didn't net one of the big ones on the hunt, hours later another hunting team caught a 741 pounder, another record. Don't mistake these for calm waters. Out here it's jurassic park under the mississippi river. Gio benitez, abc news. You can see the full story

This transcript has been automatically generated and may not be 100% accurate.

Pennsylvania Teens Capture 5-Foot-Long Gator

A group of young fisherman spotted the alligator in a creek.

{"id":20206736,"title":"Giant Alligators Weighing More Than 700 lbs. ","duration":"1:29","description":"Gio Benitez travels to the swamps of Mississippi to investigate how these creatures grew so big.","section":"WNT","mediaType":"Default"}